This invention relates to water-based organic coatings and more particularly, to such coatings which comprise the reaction product of a polyester resin with an aminoplast resin.
With the increasing shortage of hard woods such as walnut and oak, there is an increasing trend toward manufacturing wood grain panels by applying an organic coating with a simulated wood grain to inexpensive wood materials such as plywood and masonite. This procedure comprises applying a base coat to the, e.g., plywood panel, usually after the latter is filled; printing the desired wood grain pattern onto this base coat; and then applying a top coat to protect the wood grain pattern.
Presently-employed base and top coats are usually formed from a polyester which is reacted with an amino or similar resin. However, these coatings are produced from the aforementioned constituents using substantial amounts of solvents other than water. The amount of such solvents can be considerable. For example, as much as two tons of solvent may be employed to make one thousand gallons of coating material. Since most of this solvent is lost during production or thereafter in the drying of the coating, a considerable amount of pollution is caused by the escape of these fumes into the atmosphere. For this reason, it is desirable to substitute water for substantially all of the solvents previously employed.
In addition to it being desireable to employ heat-curable, water-based resin solutions, it is desirable to include water-soluble aminoplast resins, e.g., urea-formaldehyde resins, in such solutions since the latter cure to form hard, water-insoluble coatings. However, unless modified, the cured aminoplast resins are brittle or friable. For this reason, they are unsuitable, without modification, for applications where any flexibility is required, particularly if substantial temperature changes are also encountered.
Heretofore, polyester-modified aminoplast resin coatings have been made as disclosed for example in U.S. Pat. Nos. 3,477,976, 3,223,666 and 3,108,083. However, a disadvantage of these coatings is that the amount of polyester resin present in these coatings is substantially greater than the amount of aminoplast resin. Characteristically, weight ratios of polyester to aminoplast resin in the range of 3:1 to 10:1 are disclosed as being used. The low amounts of aminoplast resin employed means that the advantageous properties of these resins are being substantially modified together with the disadvantageous properties. That is, extent of cure and water and chemical resistance are reduced as compared to the aminoplast alone. Additionally, costs are increased because of the higher cost of the polyester resins.